I wasn't around then - I was a loons and cheesecloth girl - but if I had worn those skirts as a teenager it would have been to show off my waist and bum, not to hide anything.
Those were the days, [sigh].
Regardless, they would cause no harm, unlike breast binders, which can cause long and short-term problems:
Common side effects of chest binding
76-78% of people in studies reported skin/tissue problems, like tenderness, scarring, swelling, itching, infections
74-75% of people reported pain in chest, shoulders, back or abdomen
51-52% of people reported respiratory problems (like shortness of breath)
47-49% of people reported musculoskeletal symptoms, like postural changes, muscle wasting, or rib fractures (2,3)
from Clue, which seems to be an advisory service for young women.